Graphene refers to a two-dimensional planar sheet of carbon atoms arranged in a hexagonal benzene-ring structure. A free-standing graphene structure is theoretically stable only in a two-dimensional space, which implies that a planar graphene structure does not exist in a free state, being unstable with respect to formation of curved structures such as soot, fullerenes, and nanotubes. However, a two-dimensional graphene structure has been demonstrated on a surface of a three-dimensional structure, for example, on the surface of a SiC crystal.
Structurally, graphene has hybrid orbitals formed by sp2 hybridization. In the sp2 hybridization, the 2s orbital and two of the three 2p orbitals mix to form three sp2 orbitals. The one remaining p-orbital forms a pi-bond between the carbon atoms. Similar to the structure of benzene, the structure of graphene has a conjugated ring of the p-orbitals which exhibits a stabilization that is stronger than would be expected by the stabilization of conjugation alone, i.e., the graphene structure is aromatic. Unlike other allotropes of carbon such as diamond, amorphous carbon, carbon nanofoam, or fullerenes, graphene is not an allotrope of carbon since the thickness of graphene is one atomic carbon layer i.e., a sheet of graphene does not form a three dimensional crystal. However, multiple sheets of graphene may be stacked. A typical graphene “layer” may comprise a single sheet or multiple sheets of graphene, for example, between 1 sheet and 10 sheets.
Graphene has an unusual band structure in which conical electron and hole pockets meet only at the K-points of the Brillouin zone in momentum space. The energy of the charge carriers, i.e., electrons or holes, has a linear dependence on the momentum of the carriers. As a consequence, the carriers behave as relativistic Dirac-Fermions having an effective mass of zero and moving at the effective speed of light of ceff≅106 m/sec. Their relativistic quantum mechanical behavior is governed by Dirac's equation. As a consequence, graphene sheets have a large carrier mobility of up to 60,000 cm2/V-sec at 4K. At 300K, the carrier mobility is about 15,000 cm2/V-sec. Also, quantum Hall effect has been observed in graphene sheets.
A perfect graphene structure consists exclusively of hexagonal cells. Any pentagonal or heptagonal cell constitutes a structural defect. It should be noted that defects in the graphene structure converts a graphene layer into other carbon-based structures such as a large fullerenes and nanotubes, etc. In particular, carbon nanotubes may be considered as graphene sheets rolled up into nanometer-sized cylinders due to the presence of defects. A fullerene, also known as a “buckyball” having a pattern similar to the pattern on a soccer ball, would be formed if some hexagons are substituted with pentagons. Likewise, insertion of an isolated heptagon causes the sheet to become saddle-shaped. Controlled addition of pentagons and heptagons would allow a wide variety of shapes to be formed.
Graphene layers may be grown by solid state graphitization, i.e., by sublimating silicon atoms from a surface of silicon carbide surface, such as a (001) surface. At about 1,150° C., a complex pattern of surface reconstruction begins to appear at an initial stage of graphitization. Typically, a higher temperature is needed to form a graphene layer.
Formation of a graphene layer on another material is known in the art. For example, single or several layers of graphene may be formed on a silicon carbide (SiC) substrate by sublimation decomposition of a surface layer of a silicon carbide material.
U.S. Pat. No. 7,071,258 to Jang et al. and U.S. Pat. No. 6,869,581 to Kishi et al. describe known properties and methods of forming graphene layers, the contents of which are herein incorporated by reference. Further, U.S. Patent Application Publication No. 2006/0099750 to DeHeer et al. and U.S. Pat. No. 7,015,142 to DeHeer et al. describe methods of forming graphene layers, the contents of which are herein incorporated by reference.
Graphene displays many other advantageous electrical properties such as electronic coherence at near room temperature and quantum interference effects. Ballistic transport properties in small scale structures are also expected in graphene layers.
Therefore, there exists a need for a field effect transistor which utilizes the advantageous properties of graphene such as large carrier mobility and a method of manufacturing the same.
Further, there exists a need for a field effect transistor that is compatible with standard complementary metal oxide semiconductor (CMOS) processing technologies and manufacturable with minimum number of deviated processing steps.